This invention relates generally to proximity sensing devices, and in particular to article location monitoring and alarm devices.
Various sensor and/or alarm devices are known in the art, and address a variety of environments and monitored events. For example, U.S. Pat. No. 3,298,012 issued to Weller discloses a ladder warning device that provides a warning when an unauthorized person attempts to climb a monitored ladder. The monitored ladder includes one or more cross members that can not be avoided by one who seeks to climb the ladder. A mercury switch associated with this cross member provides an indication that someone has manipulated the cross member and is, hence, seeking to climb the ladder. A visual or audio alarm is then provided. Even if the mercury switch is only opened temporarily, the alarm will continue to ring until either another switch is operated, or until a timing unit expires.
In U.S. Pat. No. 3,710,371 a portable security alarm and alarm system is disclosed that also makes use of a mercury switch. Once activated, the alarm continues to sound until disabled by a predetermined process, such as use of a key lock switch. The device is intended for placement upon or near an object to be monitored. Displacement of the object causes switching of the mercury switch and the sounding of the alarm.
U.S. Pat. No. 3,797,006 discloses another safety alarm system and switch that can be used with a cabinet to monitor access to the cabinet. The device includes a spring biased switch that moves to a closed position as the cabinet is opened, due to the action of the spring. In the closed position a capacitor is allowed to charge up, and if it succeeds in charging up, an audio alarm is sounded. If, however, the cabinet is closed prior to the sounding of the alarm, the capacitor will discharge and the alarm will not sound. The switch can also be moved to a third non-spring biased open position, to allow an authorized user to open the cabinet and leave it open without causing the alarm to sound.
U.S. Pat. No. 4,117,465 discloses an alarm system for vending machines. Specially placed switches monitor the closed or open status of a door associated with the machine, and cause an alarm to sound if the door is opened in an appropriate manner. If an authorized person opens the door in the proper way, by unlocking a latch associated therewith, the unlocking procedure causes the alarm mechanism to be disabled.
U.S. Pat. No. 4,117,468 discloses a sound alarm for protecting briefcases and the like. This device monitors for an unauthorized person either picking the briefcase up, or sliding it along a floor or other surface. An alarm can also be provided if the briefcase is opened without authorization.
U.S. Pat. No. 4,146,886 discloses a freezer alarm that sounds an alarm whenever the temperature within a monitored container, such as a freezer, fails to remain within predetermined limits. To save battery life, the alarm only sounds intermittently during a condition.
U.S. Pat. No. 3,932,859 discloses a warning device such as a smoke detector. This device provides means for sensing the rate of change for the monitored event to avoid false alarms due to slow changes in natural conditions, while remaining fully responsive to rapid changes indicative of the monitored event in question, such as a fire.
U.S. Pat. No. 4,276,545 discloses a door activated burglar alarm that utilizes a time delay to provide for authorized exit from and entrance to the monitored premises.
U.S. Pat. No. 4,630,449 discloses a monitoring apparatus for use in monitoring the temperature in a device like a freezer that undergoes normal operation, such as defrosting, that could be mistaken for a system failure. The device accomplishes this by providing an alarm delay that is at least equal to the normal cycle that would otherwise imitate the undesired event being monitored.
Also, U.S. Pat. Re. No. 28,754 discloses a bed egress alarm circuit that monitors when a person has removed himself from a monitored bed. The device includes an alarm delay to allow for the person to temporarily move himself in the bed without triggering the alarm.
Notwithstanding these and other known devices, there still remain a number of unmet monitor/sense/alarm needs. For example, none of the prior art devices are satisfactory for use with monitoring articles, such as hand-held writing instruments, that are removed from appropriate holding receptacles for intended use by an individual, and that should be returned to the receptacle after utilization. Most of the prior art devices teach that an alarm should immediately be sounded upon occurrence of the monitored event. If one sounds an alarm immediately upon removing the pen from the pen holder, however, one must endure the sound of an alarm during normal and intended use of the pen until it has been returned to the holder. Some of the prior art devices would even require that the alarm continue to sound after the pen has been returned. Neither of these solutions, of course, are acceptable in context. In various ways, the remaining prior art solutions are similarly unsuitable to deal with the situation described.
A need, therefore, exists for a device that will allow monitoring of removal of an object from a holding receptacle, and that will provide an alarm when, and only when, it becomes evident that the object has not been returned to the receptacle within a period of time commensurate with its intended purpose. Such a device should be simple to operate, relatively portable as may be appropriate to certain applications, effective to ensure return of the monitored object to the receptacle, and should accommodate changing needs as they may arise. Further, such a device should be relatively easy to manufacture, and relatively inexpensive to operate.